The present disclosure relates to a device and method for controlling an electric motor.
In general, a hydraulic power steering apparatus using the oil pressure of a hydraulic pump or an electronic power steering apparatus (EPS) using an electric motor is being used in the car.
The hydraulic power steering apparatus always consumes energy irrespective of the rotation of a steering wheel because the hydraulic pump being a power source assisting in power is driven by an engine.
However, in the case of the EPS, the electric motor driven with electrical energy provides steering assisting power when the steering wheel rotates and thus torque occurs.
Thus, when the EPS is used, it is possible to enhance energy efficiency in comparison to the hydraulic power steering apparatus.
The EPS uses generated torque, a vehicle speed, or a steering angle to recognize the steering intention of a driver and the operating station of a vehicle, generates steering assisting power by considering them, and then transmits the power to a steering column, rack bar, rack and pinion so that the driver may drive more safely.
The EPS may be controlled by an electronic control unit (ECU).
The ECU checks a current flowing in the electric motor in order to precisely control the electric motor driven by a steering apparatus and calculates a torque generated at the electric motor and a ripple due to the torque, namely, torque ripple. The torque ripple is generated by an error in path through which a current detected by a current sensor flows, an error in voltage applied to the current sensor, or the DC offset of the current sensor itself and due to this, there is a limitation in that the control performance of the electric motor decreases.
The DC offset is a small amount of DC current for operating the current sensor. In order for the current sensor to measure a current flowing in the electric motor, a small amount of current for operating the current sensor is needed. A current output by the current sensor is a sum of a current flowing in the electric motor to be actually found and a small amount of current needed for operating the current sensor. In this case, the value of a current flowing actually in the electric motor to be measured is distorted due to the small amount of current.
FIG. 1 is a flowchart showing how a typical electric motor control device applies a DC offset.
Referring to FIG. 1, the ECU for operating the electric motor is initialized in step S10 and the current sensor measures a current offset value flowing in each phase in step S20.
When the measuring of the current offset value is completed, the current offset value is applied so that the operation mode of the ECU for operating the electric motor is executed in step S30. The application of PWM, namely, the operation state of the electric motor is consistently is checked during the operation of the ECU and when the electric motor is not in the operation state, a pre-measured offset value is re-measured. Also, the electric motor continues to operate according to the driving mode of the ECU irrespective of the change of the offset value in step S50.
Thus, it is difficult for the ECU to measure a new offset value while the electric motor continues to operate, and even after an offset value changes, the ECU does not perform a compensation operation with a new offset value. Thus, while the ECU controls the electric motor, it continues to operate the electric motor without compensating for an offset.